1. Field of the Invention
The present invention relates to a developing device used in an image forming apparatus such as a copier, a facsimile, or a printer.
2. Description of the Related Art
A so-called vertical developing device, in which a large space housing replenishment developer and serving as a developer accommodating portion is disposed above a developing portion comprising a development roller, is disclosed in Japanese Unexamined Patent Application Publication 2001-194883, for example, as a developing device used in this type of image forming apparatus. In this developing device, the replenishment developer is supplied in an appropriate amount by falling naturally into the developing portion from a developer accommodating chamber through an opening portion provided in a partitioning member for partitioning the developer accommodating chamber and the developing portion. A rotatable agitating member for agitating the developer is provided in the developer accommodating portion above the partitioning member, and therefore developer replenishment through natural falling is performed smoothly. Further, a rotatable agitating member for mixing together the supplied developer and the developer in the developing portion is provided in the developing portion below the partitioning member.
In the developing device constituted in this manner, the partitioning member is provided close to the agitating member in the developing portion, and therefore the agitating force of the agitating member extends to the developer in the vicinity of the partitioning member, leading to an improvement in the developer agitation performance. Further, the partitioning member functions to receive the weight of the developer in the developer accommodating portion, and therefore a situation in which the powder pressure of the developer increases in the developing portion due to the weight of the developer housed in the large developer accommodating portion such that an excessive amount of developer is charged, thereby adversely affecting the image, can be avoided.
Further, in the developing portion, a supply roller supplies the developer by contacting the development roller while rotating, and at the same time collects surplus developer from the development roller. It is preferable that the collected developer does not accumulate in the vicinity of the supply roller, but instead is moved to the vicinity of the agitating member in accordance with a developer flow generated by the rotation of the supply roller, mixed together with the replenishment developer supplied by the agitating member, and then re-supplied to the development roller via the supply roller. To create this movement in the developer, it is effective to provide a collected developer storage portion forming member for forming a collected developer storage space within a fixed distance of the supply roller such that the conveyance force of the supply roller extends to all of the collected developer.
Hence, the partitioning member for partitioning the developing portion and the developer accommodating portion and the collected developer storage portion forming member for forming a storage space for the collected developer are provided in the interior of the developing device. Generally, these members are either formed integrally with a casing for housing and holding the development roller, supply roller and so on, or incorporated into the casing as separate components.
The casing is a precision component requiring a function for holding components that have a direct effect on the image quality, such as the development roller, the supply roller and a development blade, with a high degree of precision, and is typically manufactured through die injection molding. When an attempt is made to form the partitioning member, the collected developer storage portion forming member, and a bearing portion of the agitating member integrally with the casing, the structure of the die becomes complicated. When the structure of the die becomes complicated, the heat balance deteriorates, leading to deterioration in the precision with which the components that require precision are held, and as a result, the image quality is adversely affected. Furthermore, when the structure of the die becomes complicated, productivity deteriorates and costs increase. Similarly, when a method of incorporating the members into the casing as separate components is employed, productivity deteriorates and costs increase.